<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    /* flex */
    body,
    html {
      height: 100%;
      margin: 0;
      padding: 0;
    }

    .father {
      display: flex;
      height: 100%;
      /* width: 100%; */
      flex-direction: column;
      /* background-color: pink; */
    }

    .son1 {
      height: 100px;
      width: 100%;
      background-color: red;
    }

    .son2 {
      flex: 1;
      background-color: pink;
    }
  </style>
</head>

<body>
  <div class="father">
    <div class="son1"></div>
    <div class="son2"></div>
  </div>
</body>

</html>